The compression moulding of unsaturated polyesters, specifically SMC (sheet moulding compound) or BMC (bulk moulding compound), has assumed a growing importance in the motor vehicle industry, where they are used for the manufacture of functional and structural parts of motor vehicles, with particular reference to the bodywork.
The more widespread use of unsaturated polyester resins, however, is limited by the high costs of preparing and transforming the moulding masses currently available and by problems relating to the qualitative appearance of the articles made from them.
In order to achieve the desired thickening of the resin, the production of masses of unsaturated polyester resin for moulding in fact requires a period of maturation which must generally precede the moulding by at least 24 hours, whereby it is impossible to carry out the preparation of the moulding mass and the moulding itself on a production line. Moreover, the moulding masses, such as SMC, require very long transformation periods in the closed mould, of the order of 35-45 seconds per millimetre of thickness, with a consequent limitation on productivity.
Furthermore, the products made by the moulding of SMC and/or BMC often give rise to defects which necessitate finishing operations after moulding (blowing) and recovery after painting.